1. Field of the Invention
The present invention generally relates to the art of optical imaging systems, and more specifically to the correction of conical deformation created by a camera viewing through a transparent, generally conical dome or ogive.
2. Description of the Related Art
A rocket or missile is preferably provided with a streamlined nose cone or front dome to minimize aerodynamic drag and maximize the efficiency of its propulsion system. The dome is pointed at its front end, and is axi-symmetric in that its cross section in any plane perpendicular to its longitudinal axis is circular. The dome can be perfectly conical, or have a generally conical ogive shape with a gradually curved cross section in a plane passing through its longitudinal axis.
Cameras are carried by missiles for guidance, tracking and information gathering purposes. The camera must face generally forward for tracking a ground or airborne target. It is highly desirable to make the dome transparent and mount the camera on a gimbal inside the missile so that it can view forwardly through the dome at a variable angle. However, the dome creates optical aberrations which make objects viewed therethrough appear offset from their true positions.
A dome having an ogive shape such as defined by the well known Von Karman equations is desirable over a perfectly conical dome since it produces less drag. However, a curved dome such as a Von Karman ogive introduces an aberration known as "boresight error" such that a ray passing through the dome surface exits at an angle which is non-parallel to its entrance angle.
Boresight error can be eliminated by making the inner surface of the dome non-parallel to the outer surface as disclosed in copending U.S. Pat. No. 5,220,159 filed Sep. 23, 1991, entitled "REFRACTION CORRECTION FOR AXISYMMETRIC VIEWING WINDOW" by K. Friedenthal.
A second type of aberration created by viewing through a conical or generally conical ogive dome is known as "conical deformation" and is generally astigmatic. For a perfectly conical dome, there is zero refractive power in a plane defined by the longitudinal axis of the dome and the gimbal look axis, and one-dimensional diverging power perpendicular to this plane. The power is theoretically infinite at the tip of the dome, and decreases progressively with distance from the tip. For a Von Karman or other ogive having a curved shape, there is also a relatively small amount of diverging power in the plane of the dome and gimbal look axes.
The one-dimensional refraction created by a conical dome is comparable to that created by a cylindrical lens. For this reason, attempts have been made to eliminate conical deformation using cylindrical lenses. Although a portion of the deformation can be reversed (canceled or corrected) by a cylindrical lens, the refracting power of a dome is not constant with position as with a cylindrical lens, but increases toward the tip of the dome. For this reason, an unacceptable amount of residual conical deformation exists at all but a single viewing position on the dome, even if a best compromise cylinder power has been determined based on the geometry of the design.